1. in thirty-five patients, twenty-eight with classical haemophilia and seven with Christmas disease, arthropathy of the knee of various grades has been investigated by radioisotope scanning after intravenous injection of technetium, ^99mTc.

2. The abnormality of the colour scan particularly matches the clinical severity in acute haemarthrosis.

3. In patients with no clinically apparent joint disease the scan may be of value in the early detection of involvement.

4. The possible value of articular scanning in the selection of patients for treatment and in the assessment of the short and long term results is discussed.